Posted by Home Guy on April 16, 2011, 1:42 pm
harry wrote:
> > So what you're saying is this:
> >
> > Connect 2 batteries of the same voltage together in parallel to
> > the same load and each battey will supply half the current to
> > the load.
> >
> > That sounds like a really good bargain. Just by matching the
> > power companies voltage at my service input, my PV system will
> > supply half the current - always!
> No. You match the voltage and then turn it up until the full load
> current of your array is flowing in practical terms.
> There. Do you understand that?
That's exactly what I've been saying - that you "turn it up" (the
inverter's voltage output) to maximize the PV's current (I) supply into
the grid.
But everyone else (or most everyone else) is saying no - that simply
matching the grid voltage (as measured at your service connection) is
all that happens (and is all that needs to happen) for the entire PV
current (I) capacity of the PV system to be "injected" into the grid.
So now that we agree that PV systems need to raise the grid voltage if
they're going to "force" their maximal available supply capacity into
the grid, it's a moot or academic question as to what exactly their
supply situtation would be (how much current they'd supply into the
grid) if the invertors simply matched the grid voltage.
Posted by Jim Wilkins on April 16, 2011, 2:36 pm
> harry wrote:
> > > So what you're saying is this:
> > > Connect 2 batteries of the same voltage together in parallel to
> > > the same load and each battey will supply half the current to
> > > the load.
> > > That sounds like a really good bargain. Just by matching the
> > > power companies voltage at my service input, my PV system will
> > > supply half the current - always!
> > No. You match the voltage and then turn it up until the full load
> > current of your array is flowing in practical terms.
> > There. Do you understand that?
> That's exactly what I've been saying - that you "turn it up" (the
> inverter's voltage output) to maximize the PV's current (I) supply into
> the grid.
> But everyone else (or most everyone else) is saying no - that simply
> matching the grid voltage (as measured at your service connection) is
> all that happens (and is all that needs to happen) for the entire PV
> current (I) capacity of the PV system to be "injected" into the grid.
> ...
You can only guess at what the inverter is forcing and sensing without
looking at the schematic and uP code or a technical explanation of it.
The Wiki type explanations are oversimplified for general readers,
engineers have better sources.
I was hired to decipher and troubleshoot several lead-acid and lithium
battery charging circuits after the designers quit, and found a couple
of different approaches in use. Generally they compared voltage and
current measurements to a model and used the result to pulse-width-
modulate the output control.
Power factor control is similar to the design issues of a grid-tie
inverter, with a large enough market to support custom ICs:
http://focus.ti.com/lit/an/slua144/slua144.pdf
jsw
Posted by krw@att.bizzzzzzzzzzzz on April 16, 2011, 6:39 pm
>>
>>
>>
>>
>>
>> > harry wrote:
>> > > > So what you're saying is this:
>>
>> > > > Connect 2 batteries of the same voltage together in parallel to
>> > > > the same load and each battey will supply half the current to
>> > > > the load.
>>
>> > > > That sounds like a really good bargain. Just by matching the
>> > > > power companies voltage at my service input, my PV system will
>> > > > supply half the current - always!
>> > > No. You match the voltage and then turn it up until the full load
>> > > current of your array is flowing in practical terms.
>> > > There. Do you understand that?
>>
>> > That's exactly what I've been saying - that you "turn it up" (the
>> > inverter's voltage output) to maximize the PV's current (I) supply into
>> > the grid.
>>
>> > But everyone else (or most everyone else) is saying no - that simply
>> > matching the grid voltage (as measured at your service connection) is
>> > all that happens (and is all that needs to happen) for the entire PV
>> > current (I) capacity of the PV system to be "injected" into the grid.
>> > ...
>>
>> You can only guess at what the inverter is forcing and sensing without
>> looking at the schematic and uP code or a technical explanation of it.
>> The Wiki type explanations are oversimplified for general readers,
>> engineers have better sources.
>>
>> I was hired to decipher and troubleshoot several lead-acid and lithium
>> battery charging circuits after the designers quit, and found a couple
>> of different approaches in use. Generally they compared voltage and
>> current measurements to a model and used the result to pulse-width-
>> modulate the output control.
>>
>> Power factor control is similar to the design issues of a grid-tie
>> inverter, with a large enough market to support custom
ICs:http://focus.ti.com/lit/an/slua144/slua144.pdf
>>
>> jsw- Hide quoted text -
>>
>> - Show quoted text -
>An unneccessary complication. Far easier and better to mount PF
>correction capacitors on individual motors.
Motors aren't the only things that need PF correction. Capacitors aren't the
only, or often the, way of doing it.
Posted by Jim Wilkins on April 16, 2011, 10:13 pm
> ...>
> > Power factor control is similar to the design issues of a grid-tie
> > inverter, with a large enough market to support custom ICs:
> > http://focus.ti.com/lit/an/slua144/slua144.pdf
> > jsw-
> An unneccessary complication. Far easier and better to mount PF
> correction capacitors on individual motors.-
An incorrect assumption. Those are for more complex loads like
switching power supplies with rectifier inputs.
jsw
Posted by g on April 12, 2011, 5:39 pm
On 12/04/2011 07:33, Home Guy wrote:
> g wrote:
>>
>> The grid can be seen as a pretty rigid beast. No small puny inverter
>> in the sub 1000kW class will much affect the grid voltage as a whole.
>> When voltage of the converter is attempted to be raised, current will
>> flow into the grid of course. The voltage increase will hardly be
>> measurable, as electrical characteristics of the grid will adjust
>> dynamically.
>>
>> At any one time, there is a certain load on the grid as a whole. When
>> Mr. Homeowner adds 10Kw from some solar panels, some other power
>> generating systems connected to the grid will (have to) reduce their
>> output. As a result the voltage stays the same overall.
> Here's the problem:
> Many of the load devices you find in a typical home (primarily electric
> motors that run cooling systems, air conditioners, fridges and freezers)
> are not capable of regulating their input voltage.
> So when a secondary electricity source comes on-line (like a small PV
> system) then in order to push it's current into the local grid it will
> have to *try* to raise it's output voltage in order to see some current
> flow. It might only be a few volts, maybe less.
1) The actual voltage increase will relate to the ratio of grid
impedance vs local impedance, i.e. your local power consumers (fridges,
heaters etc) has a much higher impedance relatively, thus the grid will
"take" the majority of the generated power. The _only_ increase in
voltage you will see results from the voltage drop in the grid components.
> But does that mean there will be a measurable net reduction in the
> current being supplied by the high-voltage substation for that corner of
> the city?
2) Pretty complex calculation, but yes, _somewhere_ one or more
generating pieces of machinery will reduce its output. Makes sense
intuitively, does it not?
> Not if your typical load device in homes surround the PV system will
> simply operate at a higher wattage.
3) You just set your PV system to operate at max power, the grid system
will balance out automatically. See 1) above
> The only sort of load that can effectively be regulated by a slight
> increase in local grid voltage are electric heaters. When you raise
> their input voltage slightly, they will put out more BTU of heat, and if
> their heat output set-point doesn't change, then their operational duty
> cycle will change slightly.
4) The grid voltage does actually fluctuate a bit, depending on load.
Power companies have means of adjusting line voltages depending on load
fluctuations. The average subscriber never knows this.
> The only way that a neighborhood PV system can actually supplement
> municipal utility power is when the PV system is wired up as a dedicated
> sole supply source for a few select branch circuits.
5) That will be a very inefficient way to utilize your PV system.
A simplified way is to look at the grid as a battery. When your PV
system generates more power than your local consumers, the surplus will
flow into the grid. At all other times the grid and the PV will both
supply the needed power to the local consumers.
>The way I see it,
> you have to feed certain select loads 100% from a PV system (ie -
> disconnect them from the municipal energy source) if you're going to
> make a meaningful contribution to the supply-side of a municipal or
> city-wide grid.
6) Fairly close to impossible. How do you match local power consumers to
hit the 100% PV capacity?
> >
> > Connect 2 batteries of the same voltage together in parallel to
> > the same load and each battey will supply half the current to
> > the load.
> >
> > That sounds like a really good bargain. Just by matching the
> > power companies voltage at my service input, my PV system will
> > supply half the current - always!
> No. You match the voltage and then turn it up until the full load
> current of your array is flowing in practical terms.
> There. Do you understand that?